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At least 55 records · Page 3Linked to original sources
PubMed1993

A computer-based servo system for controlling isotonic contractions of muscle.

We have developed a computer-based servo system for controlling isotonic releases in muscle. This system is a composite of commercially available devices: an IBM personal computer, an analog-to-digital (A/D) board, an Akers AE801 force transducer, and a Cambridge Technology motor. The servo loop controlling the force clamp is generated by computer via the A/D board, using a program written in QuickBASIC 4.5. Results are shown that illustrate the ability of the system to clamp the force generated by either skinned cardiac trabeculae or single rabbit psoas fibers down to the resolution of the force transducer within 4 ms. This rate is independent of the level of activation of the tissue and the size of the load imposed during the release. The key to the effectiveness of the system consists of two algorithms that are described in detail. The first is used to calculate the error signal to hold force to the desired level. The second algorithm is used to calculate the appropriate gain of the servo for a particular fiber and the size of the desired load to be imposed. The results show that the described computer-based method for controlling isotonic releases in muscle represents a good compromise between simplicity and performance and is an alternative to the custom-built digital/analog servo devices currently being used in studies of muscle mechanics.

Animals↗
PubMed1971

Effects of abrupt load alterations on force-velocity-length and time relations during isotonic contractions of heart muscle: load clamping.

1. Abrupt alterations in load (load-clamping) have been imposed on cat papillary muscles during the course of isotonic shortening, between the onset of shortening and peak shortening.2. For any given total load, whether imposed during the course of shortening or before stimulation, the velocity of shortening is determined solely by the instantaneous length, and not by the sequence of length and tension changes through which it arrived at that length.3. This unique force-velocity-length relation is independent of time from just after the onset of shortening until just prior to peak shortening.4. These results suggest that a steady state exists for the maximum intensity of active state in heart muscle over a major portion of the time during which isometric force is rising, and that heart muscle always senses total load while shortening.

Animals↗
PubMed1984

Isotonic contraction of temp-step activated muscle fibers with varied tonicity: effects of cell volume and the degree of activation.

These studies on intact fibers describe the effects of calcium, ionic strength and volume on the contraction properties. The results provide firm evidence that cell volume affects the speed but not the force. On the other hand, sarcoplasmic ionic strength affects the force development, with no effect on unloaded speed of shortening. These results suggest that there are essential differences in the rate limiting steps for isometric and isotonic properties of the cross-bridge mechanism. The studies at various degrees of activation indicate that Ca acts as a simple "on-off" switch for cross-bridge activation, in intact fibers.

Animals↗
PubMed1981

Isotonic contraction of skinned muscle fibers on a slow time base: effects of ionic strength and calcium.

The force development by calcium-activated skinned frog skeletal muscle fibers and the motion on a slow time base after a quick decrease in load were studied at 0-1 degrees C as a function of the ionic strength and the degree of activation. The ionic strength was varied between 50 and 190 mM by adding appropriate concentrations of KCl to the bathing solution. Under these conditions, the fibers could be maximally activated for several cycles at low ionic strength without developing residual tension. We found that the steady isometric force in fully activated fibers linearly decreased when the KCl concentration was increased from 0 to 140 mM. The steady isotonic motion at a given relative load in fully activated fibers was almost the same at KCl concentration greater than or equal to 50 mM. In 0 and 20 mM KCl, the isotonic velocity decreased continuously for more than 300 ms. At a given relative load, the initial velocity of the motion in 0 and 20 mM KCl was about 0.6 and 0.9 times, respectively, that in 140 mM KCl. The initial velocity decreased further when residual tension developed; this observation provides additional evidence that residual tension may reflect the presence of an internal load. The effect of calcium on the motion was examined at 70 mM KCl. In this solution, the motion during the velocity transient at a given relative load appeared to be the same at different levels of activation. The speed of the subsequent motion was almost steady at high calcium levels but decreased continuously in low calcium levels. These results support the idea that at low ionic strength the response of the fiber to calcium is switch-like, but that other factors also affect the contraction mechanism under these conditions.

Animals↗
PubMed1981

Human motor unit activity during the onset of muscle fatigue in submaximal isometric isotonic contraction.

The onset of fatigue has been studied by relating motor unit activity and surface myoelectric activity during constant force isometric contraction in man. The surface (global) EMG of the biceps brachii was recorded using bipolar electrodes. The global EMGs of the other elbow flexors were also recorded. Motor unit activity was recorded simultaneously at three points of the biceps brachii by using wire electrodes. The time course of the integrated surface EMG showed that fatigue occurred right at the beginning of the contraction. The increase of the integrated EMG was especially important when the value of the force maintenance was high. The time course of this increase was close to that of the output of the spikes recorded by the wire electrodes. No matter what the value of the force maintenance, MU recruitment occurred throughout the contraction. The existence of an increase in the MU firing frequently depended on the value of this force. Contractile element fatigue is believed to provide the explanation of these results, while neuromuscular junction fatigue may perhaps be more important near the time limit.

Action Potentials↗